This invention is particularly adapted to ribbon drives for impact printers. Such impact printers can be dot matrix printers and more specifically line type printers. Such line printers are known in the art and have been developed extensively by the assignee of this invention.
The ink ribbons of such various printers are repeatedly impacted against a length of print paper or other printable medium by certain impact elements. The impact elements may each define a shape or a character in the case of a full character printer. Alternatively, the impact elements can simply print dots, with characters or other indicia to be printed being formed in a dot matrix fashion.
Such dot matrix printers can be of the serial type in which a print head containing a number of print wires, needles, or other dot forming elements is reciprocated across a width of printer paper. The print wires are selectively actuated to impact the print paper through a length of ribbon to print dots on the paper.
Dot matrix printers can also be of the line printer type in which a plurality of hammers or other impact printing mechanisms are mounted along the length of a hammerbank. They are driven by a shuttle assembly and are selectively actuated to impact the print paper. The impact is through a length of ink ribbon. This prints dots on the paper as the shuttle assembly is caused to undergo reciprocating motion relative to the paper. These printers can be exemplified by way of example of a dot matrix line printer in U.S. Pat. No. 3,941,051, entitled Printer System, by Barrus, et al. It is commonly assigned with the present application.
The printers of the type described in U.S. Pat. No. 3,941,051 to Barrus et al utilizes a ribbon drive. The ribbon drive has an opposite pair of spools disposed adjacent opposite ends of a print station. This is defined by the interface between a platen supported length of print paper and a reciprocating hammer bank.
The ink ribbon extends through the print station and has the opposite ends thereof wound upon an opposite pair of spools. During printing, the opposite spools of the ribbon drive are rotatably driven to provide continuous motion of the length of ribbon through the print station. When the end of the ribbon is reached the direction of drive of the spools is reversed. This causes the ribbon to move through the print station in the opposite direction.
A major problem for all printers is the drive of the spools. An example of this problem is exemplified in U.S. Pat. No. 4,177,731 commonly assigned to the assignee of this application. In the prior art, the drive of the spools was often by means of a D.C. or other continuous motor. The motors turned in a manner and were driven so as to not accurately account often times for the change of radius of the unwinding or winding of the ribbon on the spools. This adversely affected the velocity of the printer ribbon across the print station.
The foregoing action created problems associated with ribbon usage, as well as the quality of print.
This invention overcomes many deficiencies of prior art ribbon drives by using a pair of two phase stepper motors. The stepper motors control the tension and the velocity of the print ribbon between the two spools. At any given time during printing, one motor pulls ribbon through the print station at a constant velocity while the second motor is run in a regenerative mode to provide tension within the system's operational requirements. When the ribbon reaches the end of it's travel in any given direction, it's direction is reversed so that the function of the two respective motors and spools are then reversed.
The angular velocity of the feed motor or motor attached to the spool from which the ribbon is unwinding, is calculated by counting the number of zero crossings on the back EMF waveform. In any given period of time the angular velocity of the take-up motor is set by the system micro controller. The ratio of the angular velocities of the feed take-up spool or reel is a unique number over the range of ribbon radii in the system.
The ribbon stretch is insignificant over the normal life of the ribbon as well as the packing factor on the take-up reel or spool upon which the ribbon is wound or pulled. Thus, the radius of the ribbon material on each reel can be determined at set intervals based upon the ratio of the angular velocities. This ratio information used to determine the angular velocity and tension is updated at any given time.
The above method allows a set of micro-step interval timer tables to relate the ratio of the angular velocities from the last sample interval to the next required angular velocity on the take-up spool or reel to maintain a constant linear velocity. The invention provides the ability to control tension as well as velocity which are independently controllable variables. It should be kept in mind that substantially constant linear velocity is one of the key results of this invention. With this ability, the finite characteristics of the ribbon velocity thereby become a more readily ascertainable factor as to the characteristics of ribbon wear, print, dot ratios, spacing, and general overall print quality.
The invention also accounts for jams and tearing of the ribbon. This helps to avoid a catastrophic failure out of the normal sequence of the printer's operation.
To effectuate the invention, a take-up two phase stepper motor or winding motor drive with a take-up spool or reel is driven by a pair of voltage mode pulse width modulation (pwm) amplifiers. One is for each phase of the respective two phase stepper motor. The average voltage across the motor terminals is proportional to the duty cycle at a given supply voltage. The micro controller determines the value of the duty cycle at a given angular velocity for each micro-step from a set of look up tables. Instead of a look up table the value can also be calculated by an on going real time calculation.
An object of this invention is to maintain a constant linear ribbon velocity across the print station.
Another object is to provide the ability to change ribbon velocity on "the fly" to accommodate different print modes.
A further object is to create a system with the ability to accommodate different standard lengths of ribbon materials.
Another object of the invention maintains a constant ribbon tension tracked independently of the ribbon velocity.
Finally a substantially important object of the invention is the ability to detect all fault modes and report it to the main system controller.
From the foregoing, it can be seen that the inherent improved characteristics of the ribbon drive with the attendant superior ribbon performance established by the stepper motor drive of this system is a significant step over the art which shall be expanded upon hereinafter.